This invention relates to controlling a high frequency link cycloconverter system, and more particularly to a current controller and method for operating a cycloconverter to control real and reactive power flow to and from a high frequency resonant tank circuit.
The high frequency link cycloconverter system originally disclosed in U.S. Pat. No. 3,742,336 to B. D. Bedford and also in U.S. Pat. No. 3,882,369 to W. McMurray, both assigned to the same assignee as this invention, is essentially comprised of two cascaded cycloconverters separated by a high frequency parallel resonant tank circuit which provides variable commutation energy for both cycloconverters. Normally, the input cycloconverter operates in the inverting mode and the output cycloconverter in the rectifying mode, although during regeneration their roles are reversed. For static VAR control applications, the output cycloconverter is not required.
The input cycloconverter is supplied with 60 Hz or other low frequency polyphase voltage through a series reactor in each phase and converts the a-c polyphase voltage directly to the high frequency tank. The input cycloconverter can be considered analogous to a synchronous machine operating on infinite bus and capable of controlling real and reactive power independently or real power and input power factor independently. One control strategy for accomplishing this for both directions of power flow is based on controlling the in-phase and quadrature components of induced cycloconverter voltage, which is the phasor summation of the line voltage and the voltage across the input reactor in each phase. This is further explained in copending application Ser. No. 573,373, filed on Apr. 30, 1975 by Bimal K. Bose and the inventor, now U.S. Pat. No. 3,959,720, entitled "Voltage Control System for High Frequency Link Cycloconverter," and the application of the voltage control system to a static VAR controller is disclosed in the concurrently filed application Ser. No. 573,372, by the inventor, now U.S. Pat. No. 3,959,719, entitled "Static Controller for Power Factor Correction and Adaptive Filtering," both assigned to the assignee of this invention.
In any control strategy it is necessary to regulate the tank voltage to be relatively constant in view of the fact that the high frequency tank is a small reservoir of energy compared to the power rating of the whole circuit. Although satisfactory for many applications, a possible deficiency of the voltage control system is that the time constant for the series reactors in the 60 Hz lines restricts the bandwidth of the high frequency tank voltage regulator. High static and transient loading on the system can cause the tank voltage to modulate at tank frequency plus or minus even multiples of the load and source frequency. These modulation frequencies are beyond the bandwidth of the tank voltage regulator. Since this modulation can deteriorate the input cycloconverter commutation capability, compensation can increase power circuit costs to maintain a given minimum tank voltage.